The present invention is generally related to processes for metallizing substrates, and more particularly to a starved-reaction metal-oxide reduction process for metallizing substrates having flat or three-dimensional surfaces.
A two-step dielectric process for manufacture of multilayered circuits (described in the instant assignee's copending U.S. patent application Ser. No. 07/461,857 entitled "Dielectric Layered Sequentially Processed Circuit Board", filed Jan. 8, 1990, and invented by Vernon L. Brown) requires the sequential deposition and photo-definition of openings in two resins. The first resin, called resin A, is filled with a material referred to herein as a filler, which when activated, will promote electroless plating with a metal. The second resin, called resin B, does not contain such a filler. When each resin, in turn, is photochemically patterned, resin B over resin A, openings produced in resin B define areas called "channels"; superimposed openings produced in resin A and resin B define deeper areas called "wells" which open to a metal layer below. When this structure is subjected to processes which modify or activate particles contained in or at the surface of resin A, the channels and wells may be coated with an electrolessly plated metal to form metallic features, such as pads, vias and conductors.
According to another process described in U.S. Pat. No. 4,590,115, a metal oxide, typically cuprous oxide, is the filler of choice for plating plastics, which are permeated with small particles of such cuprous oxide. Cuprous oxide becomes a catalyst through chemical reduction. This metal-oxide reduction process uses immersion in a borohydride solution to reduce the small cuprous oxide particles to metal both on and below the surface of the plastic substrate. While this metal-oxide reduction process exhibits very rapid and excellent copper coverage on the substrate surface, the process is not suitable for selective plating, such as, for example, the two-step dielectric process described hereinabove, since copper growth can not be confined to the defined openings in resin B. The undesired over-plating that results from this metal-oxide reduction process is due to the use of a large concentration of cuprous oxide in the resin and the use of a film or bath containing a large amount of the borohydride reducing agent, both of which promote uncontrolled metal film growth. Another problem, when using this metal-oxide reduction process in the two-step dielectric process described hereinabove, is that the dielectric constant of resin A, permeated with many small cuprous oxide particles, becomes relatively high due to the required large amount of cuprous oxide (e.g., 25-90% by weight). Furthermore, the large amount of cuprous oxide also interferes with photo-patterning and complicates resin coating, resin surface etching, and image development. Therefore, for the foregoing reasons, there is a need for an improved reduction process for selectively metallizing substrates without degrading the electrical and physical characteristics of the substrate.